1. Field of the Invention
This invention relates to gold solders for use in soldering such ornamental parts as watch cases composed of materials such as a gold alloy, stainless steel, stellite, sintered carbide, or the like.
2. Prior Art
Ornamental parts made of the above materials each exhibit excellent corrosion resistance. Therefore, when a plurality of parts are joined by soldering each other, the solder to be used is also required to have excellent corrosion resistance. In such a case, gold solders of Au-Cu-Ag alloys or Au-Cu-Ni alloys containing 50% (equivalent to 12 Karat) or more gold have been used. They have a melting temperature around 850.degree. C. However, when mirror-polished ornamental parts are heated at high temperatures for soldering, their surfaces are often coarsened by recrystallization. After the soldering operation, it is necessary to repolish them, but some shapes are often incapable of repolish.
Table I shows the degradation states caused when various metal surfaces previously mirror-polished by chromic oxide or diamond powders are heated at high temperatures.
TABLE I ______________________________________ Degradation of Mirror-Polished Metal Surfaces Heated at High Temperatures* Stel- 18Kt Temp. Time SUS304 lite TaC WC Au ______________________________________ 750.degree. C. 15 No No No No No min change change change change change 60 No Blurred No Blurred Pear min change change surface 800.degree. C. 3 No No No No No min change change change change change 15 Blurred Blurred No Blurred Pear min change surface 60 -- -- Blurred -- -- min ______________________________________ *heated in a hydrogen atmosphere (dew point -70.degree. C.)?
As shown in Table I, in the range of 15 minutes at 750.degree. C. or 3 minutes at 800.degree. C., it is possible to heat the metal surfaces of the 18 Karat gold alloys, stainless steels (SUS304), stellites (Co-base alloy), sintered carbides (TaC and WC) without degrading their mirror-polished surfaces. For this reason, a gold solder having a melting temperature up to 800.degree. C. is desirable.
When Zn, In, Sn and Pb are added to a gold solder containing 50% or more gold, they are known to serve as the elements for lowering the melting temperature of the gold solder. However, the addition of these elements is to a disadvantage in degrading the corrosion resistance and the toughness of the gold solder. Furthermore, since the vapor pressures of the solder becomes high, the vicinities of joined portions are contaminated.
The purpose of the invention is to provide a gold solder having a melting temperature of 800.degree. C. or below without impairing the excellent corrosion resistance and toughness of the conventional gold solder containing 50% or more gold.